According to current technology, the most basic approach for using oxygen to burn coal accepts the high flame temperatures produced and arranges the furnace to remove the coal ash constituents as a running slag. This approach is impractical to retrofit to most pulverized coal furnaces which are configured to maintain the coal ash constituents above the ash softening temperature; hence they are known as dry-bottom furnaces.
The current technology seeks to implement oxygen-coal combustion in dry-bottom furnaces by controlling the oxygen-fuel flame so as to maintain peak flame temperatures close to those experienced under normal conditions when coal is burned in air. This ensures that the slagging and fouling behaviour of the coal ash constituents is maintained such that the deposits in the furnace and various heat transfer surfaces of the boiler can be removed effectively using conventional removal techniques.
The current art of oxygen-fuel combustion favours recycling large quantities, for example over 50%, of the total products of combustion in order to accomplish the above mentioned flame control. This recycling of relatively cold flue gases to the furnace also helps to maintain the flue gas weight through the boiler close to that experienced under normal conditions when coal is burned in air, thereby maintaining adequate convective heat transfer to the boiler surfaces.
The current technology used for oxygen-fuel combustion favours the pre-mixing of oxygen into the flue gas stream delivered to the furnace in order to convey the oxygen to the flame. This approach necessarily considers pre-mixing oxygen into several streams, such as the primary stream used to dry and convey the coal to the furnace, a secondary stream used by the burner and finally a tertiary stream used by the burner and/or over-fire system if separate from the burner.
An important safety consideration when pre-mixing oxygen with these various known flue gas streams is the desirability of maintaining the oxygen concentration below 28% by volume (dry basis), and preferably of maintaining the oxygen concentration below 25% in order to provide a safety margin for operational upsets. Achieving a 25% oxygen concentration when constrained to pre-mixing all of the oxygen required leads to yet another reason for recirculating relatively large quantities of cold flue gas to the furnace.
The current techniques of oxygen-fuel combustion can be improved by choosing to introduce a component of the total oxygen demand directly at the burner in relatively pure form. This approach however, is complicated by the interaction of the oxygen jets so created with and within the flame envelope. While the use of relatively high momentum oxygen jets to induce recirculation patterns near the burner and in the furnace may be of interest for firing gaseous and liquid fuels, or even solid fuels of exceptional quality, defined as having low ash content and high volatile content, the use of such oxygen jets in dry bottom pulverized coal fired furnaces can work against the flow field established in the furnace by the action of the burners in such a way as to create poor combustion and unsuitable accumulation patterns for the coal ash constituents both on the burner and on the furnace walls in general.
To summarize, the current technology of oxygen-fuel combustion incurs significant complexity and cost to retrofit an existing boiler in order to recirculate relatively large quantities of cold flue gas to the furnace, in particular for conditioning and transporting the flue gas stream that is recycled to the furnace for the purpose of acting as the oxygen carrier stream. Conditioning of this flue gas stream is necessary to remove particulates, to cool the flue gas stream in order to preserve the operation of the air-heaters and to be able to move the recycled stream through existing ductwork.
The current art of oxygen-fuel combustion also requires that significant modifications be made to the existing combustion system, including but not limited to; modifying the burners to establish proper flow areas, arranging for pre-mixing the oxygen into the required streams and incorporating any oxygen jets that may be required into the system.
In order to overcome these difficulties with the known systems, this invention proposes a new configuration for an oxygen-fuel combustion system, which is both relatively simple and easily incorporated into new or existing boilers in an economical manner. The concept includes a preferred process configuration, a preferred method for introducing oxygen to the furnace and a preferred method of introducing coal to the furnace. This invention therefore seeks to provide a new process for oxygen-fuel combustion using a novel method of introducing oxidant and fuel to the furnace.